Although the existence of a round, hollow, geodesic-sphere shaped molecule consisting of 60 carbon atoms was first proposed in 1985 by Kroto et al. (Nature, Vol. 318, p. 162, 1985), it was not until 1990 that measurable amounts of this substance were prepared by Kratschmer et al. (Nature, Vol. 347, p. 354, 1990). This molecule was later named buckminsterfullerene or fullerene in honor of Buckminster Fuller, the inventor of the geodesic dome.
This form of carbon was obtained by resistive heating of graphite rods in an inert helium atmosphere. It is now known that fullerenes may be produced even from coal (Dance et al. J. Phys. Chem., 95, p. 8425, 1991), a cheaper alternative to the graphite process. It has also been determined that, besides the normally occurring carbon soot contaminants such as benzene, anthracene and other polynuclear aromatics, a variety of other different carbon complexes exist, including less round, yet hollow, molecules such as C.sub.32, C.sub.50, C.sub.70, C.sub.84, and other fullerenes even larger than C.sub.960. This new form of carbon complements the well known pyramidal shape of diamond carbon, and the hexagonal shape of graphite sheets.
The reported stability and reactivity of these complexes opens broad avenues for new applications and products such as superconductors, high-temperature lubricants and catalysts. C.sub.60 is reported to be stable to pressures up to 2.5.times.10.sup.6 psi (Yoo and Nellis, Science, 254, p. 1489, 1991) and, like carbon graphite is stable in many organic solvents for several weeks. High resonance energy of C.sub.60 and C.sub.70 brought about by their conjugated double bonds is responsible for their enhanced molecular stability and strength.
Simple resistive heating yields less fullerenes than vaporization by plasma arc or Nd:YAG laser vaporization. In the plasma arc method, the two rods are brought to touch each other to strike an arc and then are separated to a distance where maximum plasma brightness occurs. Using this method, yields of soluble material, mainly fullerenes C.sub.60 -C.sub.266, of up to 44% are obtainable.
Mixtures of fullerene complexes in carbon soot as prepared by a contact arc method are available from MER Corporation of Tucson, Arizona. By far the most abundant of all fullerenes in the raw soot are C.sub.60 and C.sub.70, with a ratio C.sub.60 /C.sub.70 of about 7/1. However, fractional content of these compounds in the carbon soot obtained by this method can vary widely, between approximately three (3) and 33 percent by weight. It is thus highly desirable to find a separation technique that provides substantially pure fractions of C.sub.60 and C.sub.70 in order to expand the use of such compounds.
Conventionally, solutes in solid matrices are recovered by either liquid extraction or sublimation. Liquid extraction generally consists of exposing a quantity of the solid matrix to a liquid solvent for a period long enough to transfer most soluble material into the liquid phase. Solvents such as hexane, benzene, and toluene are currently used for extraction. Selective extraction of C.sub.60 and C.sub.70 from the soot is typically obtained by extraction with hexane.
Extraction of fullerenes by sublimation essentially involves evaporation of the volatile material under vacuum and/or in an inert atmosphere into a gas phase maintained at a temperature above the sublimation temperature of the desired fullerenes (400.degree.-600.degree. C. for C.sub.60). The fullerenes are then recovered in nearly pure form on separate regions of a cooled collector.
While liquid extraction is most efficient at recovering the bulk of the extract, it is non-selective, time consuming, and generally requires further cleanup and fractionation of the extract.
Another extraction method known in the art is supercritical fluid extraction (SFE). SFE makes use of supercritical fluids (SCFs) as extraction solvents, i.e. fluids at temperatures and pressures above their critical temperature and critical pressure. SCFs are neither gaseous nor liquid, but rather exhibit intermediate properties between gas and liquid properties. However, while SCF densities, i.e. solvent power, are comparable to liquid densities, SCF transport properties such as viscosity and diffusivity are essentially halfway between gas and liquid properties. As a consequence of the combined high solvent power and high diffusivity of SCFs, SFE is relatively rapid, selective, and efficient, and its operating conditions of temperature, pressure, and nature of extraction solvent can be adjusted to quantitatively extract a variety of analytes from solid or liquid matrices. Furthermore, when appropriate, SFE can be conducted with CO.sub.2 as a SCF. Besides its mild critical temperature and pressure (T.sub.c =31.degree. C., P.sub.c =1070 psi), CO.sub.2 is non-toxic, inexpensive and easily removed from the extract by pressure reduction. However, while CO.sub.2 is the most widely used SFE solvent, its lack of polarity and aromaticity limit its utility as SFE solvent to small or relatively non-polar and hydrophobic molecules, and is thereby not expected to recover quantitative amounts of fullerenes.
Following extraction by either method, pure C.sub.60 and C.sub.70 are obtained by separation from other extracted material. Separation of C.sub.60 from low molecular weight compounds and higher fullerene extracts on an alumina column was reported by Diedrich et al. (Science, in print). In this method, the extract is first adsorbed on neutral alumina, and then chromatographed on the same alumina column with a mixture of 5% hexane in toluene to elute C.sub.60, and 10-50% hexane in toluene to elute C.sub.70. In the same manner, C.sub.60 can be separated from a fullerene mixture on a graphite column (Vassalo et al., J. Chem. Soc. Chem. Commun., p. 60, 1992), but recovery of C.sub.70 and higher fullerenes is done by soxhlet.